An error in a calibration gas may cause errors in thousands of analytical results. Therefore verification of calibration gases has become a key issue in gas metrology. As a prominent example ISO 6142-1 requires that the “preparatory composition” of gravimetric gas mixtures is checked analytically. In addition, for calibration gases used as reference materials, stability monitoring is mandatory.
Two approaches for such purposes were initiated in ISO 6143 and further developed in the framework of a German standardisation project:
(a) Univariate approach: A single calibration gas is analysed using a validated method. For each component the difference between assigned content and analytical result is examined for significance against the associated uncertainty. Agreement provides confirmation that the assigned content and its uncertainty are valid. In further development the focus was on performance measures characterising the sensitivity of the test and the level of confirmation provided by positive test results.
(b) Multivariate approach: Entire suites of calibration gases are measured in a calibration experiment, using an analyser with linear response. For each component the calibration data are examined for compatibility with a straight-line response curve. A positive test result provides confirmation that the assigned contents and their uncertainties are mutually consistent. By extension to sub-suites and combination with cross-validation this approach was developed into a toolbox for outlier search in suites of calibration gases.
The main target groups for these tools are specialty gas manufacturers and analytical laboratories. Application fields include
– Process control in the production of calibration gases
– Checking in-house prepared calibration gases
– Stability monitoring for suites of calibration gases of varying age
The results of the German standardisation project, including worked examples on MS Excel worksheets and a computer program as MS-Excel Add-In, are available from a technical report.
Bert is an Analytical Chemist and Senior Scientist in the PTB department 3.3 “Thermophysical quantities” of the PTB. He is responsible for the pattern approval of natural gas analyzers and the certification of their test and calibration gas mixtures according German verification law. He works additionally as a technical auditor for the German national accreditation authority (DAkkS) in the field of gases and is a member of the German standardization committee for gas analysis and gas quality which is the German mirror committee of ISO TC 158 and TC 193.